1. Field of the Invention
The present invention relates to a suspension apparatus of an automotive vehicle and, more particularly, to a suspension apparatus of an automotive vehicle, which is so arranged as to alter its suspension characteristics by controlling supply or discharge of an operating liquid to or from a liquid pressure chamber of a liquid cylinder disposed between a member on the vehicle body side and a member on each wheel side.
2. Description of Related Art
Heretofore, for example, Japanese Patent Laid-open Publication (kokai) No. 130,418/1988 discloses an active suspension apparatus adapted so as to alter its suspension characteristics by altering the amount of a liquid disposed in a liquid cylinder by controlling the supply or discharge of an operating liquid to or from the liquid cylinder mounted between a member on the vehicle body side and a member on each wheel side in accordance with the running state of the vehicle. This active suspension apparatus has an actuator for carrying out the supply and discharge of the operating liquid to and from the liquid pressure chamber of the liquid cylinder, an acceleration detecting means for detecting acceleration in the vertical direction, or vertical acceleration, of the vehicle body, an integration means for integrating a signal detected by the acceleration detecting means, and a control means for determining a control amount of the actuator for controlling the liquid cylinder by multiplying the output signal generated by the integration means with a predetermined gain coefficient. This active suspension apparatus is arranged such that, for instance, when it is confirmed by signals detected by the acceleration detecting means that the vehicle body is bouncing, the control signal corresponding to the control amount determined by the control means is generated to the actuator, thereby subjecting the amount of the operating liquid to be supplied or discharged to or from the liquid cylinder to a feedback control and consequently reducing the bounce of the vehicle body.
It is to be noted herein, however, that this active suspension apparatus is so arranged as to allow the operating state of the liquid cylinder to vary with the frequency of vibration acting upon the vehicle body, so that a gain of the control amount in a high frequency region is considered to be reduced by inputting the output signal of the control amount determined by the control means through a low pass filter to the actuator. In other words, it is considered that the low pass filter is so arranged as to allow the level of the control signal to be reduced in accordance with an increase in the frequency of vibration by multiplying the control amount with a value expressed by a predetermined time constant T and a Laplace's operator is so set as to increase with an increase in the frequency of vibration, i.e., by {1/(1+Ts)}. This arrangement permits an appropriate operation of the liquid cylinder by generating a control signal of such a level as corresponding to the control amount and a long-term maintenance of better comfort, when the frequency of vibration acting upon the vehicle body is small, on the one hand. When the frequency of vibration is large, on the other hand, the arrangement further suppresses the liquid cylinder from operating by reducing the level of the control signal and prevents the liquid cylinder to frequently operate followed by the vibration in a high frequency region, thereby avoiding power being wasted.
As described hereinabove, when the operating amount of the liquid cylinder is controlled by generating the control signal to the actuator through the low pass filter, it has been found, however, that the vibration acting upon the vehicle body is so deviated as to become larger upon occurrence of an oscillating phenomenon in a particular high-frequency region due to a delay of response. When the oscillating phenomenon will occur in such a state that the vibration having a large amplitude acts upon the vehicle body, the vibration of the vehicle becomes larger to the contrary, so that comfort is impaired.